1. Field of the Invention
The present invention relates to a semiconductor device having a heterojunction such as a photovoltaic device for directly converting light energy of solar light or the like into electrical energy, a thin film transistor, a bipolar transistor, or a diode and a method of fabricating the same.
2. Description of the Prior Art
As disclosed in U.S. Pat. No. 4,496,788, a heterojunction type photovoltaic device in which an amorphous silicon layer or a microcrystalline silicon layer is stacked on a monocrystalline or polycrystalline silicon substrate has been known.
A heterojunction using the crystalline silicon and the amorphous silicon layer or the microcrystalline silicon layer performs the function of joining the crystalline silicon and the amorphous silicon layer or the microcrystalline silicon layer by doping impurities into the amorphous silicon layer or the microcrystalline silicon layer.
However, the amorphous silicon layer or the microcrystalline silicon layer into which impurities are doped has the problem that the defects thereof are increased by the doping, degrading the heterojunction interface characteristics thereof. The degradation of the junction interface characteristics results in the recombination of carriers when the amorphous silicon layer or the microcrystalline silicon layer is used for the photovoltaic device. As a result, high conversion efficiency is not obtained.
In order to solve the problem, it has been proposed in U.S. Pat. No. 5,213,628 that a substantially intrinsic amorphous silicon layer is interposed between a crystalline silicon substrate and an amorphous silicon layer, thereby to reduce the defects at the interface therebetween to improve the heterojunction interface characteristics.
As the above described method, a method of depositing an amorphous silicon layer on a crystalline silicon substrate by chemical vapor deposition (CVD) or the like has been used. Therefore, the junction interface characteristics of the amorphous silicon layer depends on the cleanness on the surface of the crystalline silicon substrate before the deposition. Accordingly, careful attention must be given to the cleaning on the surface of the crystalline silicon substrate. However, some problems arise. For example, good junction interface characteristics are not frequently obtained depending on the forming conditions of the amorphous silicon layer.